Pultrusion is a highly cost effective method for making fiber reinforced resin matrix composites. The primary raw materials used in pultrusion are resin and reinforcement. Fillers and additives, such as, but not limited to, calcium carbonate, clay, mica, pigments, and UV stabilizers, may be added to the resin to enhance the physical, chemical, and mechanical properties of the pultruded product.
Pultrusion is typically done by the injection die or open bath process. The open bath process is the most common. The injection die process, however, is gaining importance due to environmental concerns about the large amounts of volatile contaminants released in the open bath process. In a typical open bath process, reinforcement material in the form of fibers, mat or roving is pulled continuously through an open bath of resin to produce an impregnated reinforcement. The impregnated reinforcement is pulled through form plates to remove excess resin, and then through a curing die to cure the resin and yield a finished product. In the injection die pultrusion process, reinforcement material is passed through a closed injection die that has resin injection ports. The resin is injected under pressure through the ports to impregnate the reinforcement material. The impregnated reinforcement is pulled through the injection die to produce a shaped product.
Resins that have been used in the open bath and injection die methods of pultrusion include thermoset resins, such as unsaturated polyesters, epoxies, phenolics, methacrylates and the like, as well as thermoplastic resins such as PPS, ABS, Nylon 6. Blocked polyurethane prepolymers also have been used. Polyester and epoxy resins are generally slower reacting than polyisocyanate-based thermosets, such as polyurethanes and polyisocyanurates. In addition, the use of blocked polyurethane resins in pultrusion has the disadvantage of requiring deblocking of the isocyanate to form a volatile by-product. This creates environmental concerns and may cause unwanted plasticization of the cured resin.
One component resin systems that are used in pultrusion include thermoset resins, which cure through ethylenic unsaturation, such as unsaturated polyesters, vinyl esters, (meth)acrylics, and the like. These types of resins generally require the use of volatile unsaturated monomers, such styrene and/or methyl methacrylate. As such, resins of this type emit volatile organic compounds (VOC's) during processing. Engineering solutions to the VOC issue, such as the use of closed injection dies, have had only limited success in controlling these emissions and the intense odors that they produce. The monomers used in the production of isocyanate-based resins are usually much less volatile than the unsaturated monomers. Accordingly, polyisocyanate-based resin systems have some inherent advantages. However, isocyanate-based formulations have had difficulties due to their relatively high reactivity at ambient temperatures.
Direct mixing activation has also been used to form polyisocyanate-based matrix polymers in the pultrusion process. Mixing activated systems of this type generally consist of a polyisocyanate component and an isocyanate reactive component (see e.g. WO 00/29459). The mixing activated systems disclosed in the prior art generally have a limited range of processability. This is due to the highly reactive nature of the mixing activated free isocyanate based chemistry. A careful balance needs to be struck between the demands of adequate mixing and fiber wetting, the achievement of economically effective line speeds, and the physical properties required in the final pultruded composite article. The ideal mixing activated resin system has a long open time (or pot life) during (and after) mixing at relatively low temperature, but is characterized by rapid and even cure at the higher temperatures used for resin curing in the pultrusion curing die.
A need therefore exists for mixing activated isocyanate-based resin systems, such as polyisocyanurate and polyurethane resin systems, that may be used in pultrusion, especially injection die pultrusion, which provide for a better combination of long pot life and fast cure.